A sensor for detecting acceleration, angular velocity, pressure or the like generally includes a transducing element for converting the amount of displacement of an object to be detected into an electric signal, and a circuit which electrically amplifies the weak electric signal output from the element for output. The known sensor signal output circuit is illustrated by FIG. 7.
In FIG. 7, first differential amplifier 26 is formed of transistors 1, 2 having their respective sources connected to each other, and constant current source 20 connected between the sources of transistors 1, 2 and first power supply terminal 33. The electric signal from the sensor is input to a gate of transistor 1, and first reference voltage setting part 28 is provided at a gate of transistor 2.
First load resistor 30 is an active load for first differential amplifier 26. This load resistor 30 is formed of diode-connected transistor 3, and transistor 4 having its gate connected to a gate of transistor 3. The gate and a drain of transistor 3 are connected to a drain of transistor 1, while a source of this transistor 3 is connected to second power supply terminal 34. Transistor 4 has a drain connected to a drain of transistor 2, and a source connected to second power supply terminal 34.
First transistor 5 for preamplification has a gate connected to the drain of transistor 2, a source coupled to second power supply terminal 34 via first constant current source 21, and a drain connected to first power supply terminal 33. The drain of transistor 2 provides an output of first differential amplifier 26.
Second transistor 6 for output has a gate connected to the source of first transistor 5, a source connected to second power supply terminal 34 and a drain coupled to first power supply terminal 33 via second constant current source 22. The drain of second transistor 6 is also connected to output terminal 32.
In this sensor signal output circuit, the sum of current flowing from the drain of transistor 3 to the drain of transistor 1 and current flowing from the drain of transistor 4 to the drain of transistor 2 is maintained, so that as the signal input to the gate of transistor 1 increases, drain voltage of transistor 2 or the output of first differential amplifier 26 increases accordingly. Since the gate of first transistor 5 is at the same potential as the drain of transistor 2, source voltage of first transistor 5 increases. This increase in the source voltage of first transistor 5 results in a decrease in drain voltage of second transistor 6, whereby output voltage of output terminal 32 decreases.
When a break in a wire or a short circuit to first power supply terminal 33 occurs in transmission of the output voltage to a receiving circuit through use of the wire, the following problem occurs.
Although the voltage input to the receiving circuit is equal to a potential of first power supply terminal 33, with only this output circuit, a determination cannot be made as to whether this input voltage is a normal output of the sensor or a voltage resulting from the break in the wire or the short circuit to first power supply terminal 33. Accordingly, another circuit has been required for detecting the short circuit.